All-optical transport nodes for optical communication networks are currently based on multi-directional-switching Reconfigurable Add and Drop Multiplexers, ROADMs, to support the use of the transport nodes in a meshed network architecture. ROADMs currently use unidirectional 1×N wavelength selective switches, WSS, that are implemented in a mechanical package using free space optics and use either a Liquid Crystal or a micro-electro-mechanical switch, MEMS, to direct each optical signal to its respective destination port. Bulk optic gratings are also used to multiplex or demultiplex optical signals. By properly combining WSS, ROADMs are capable of operating on traffic transmitted from and toward different directions, A multi directional optical switching ROADM node can be obtained by interconnecting a plurality of WSS.
The flexibility of an optical communication network is based on remotely reconfigured ROADM's. At each node of the network comprising a ROADM it is possible to route optical communication signals at by-pass wavelength from any direction to any direction, known as “directionless” routing. However at end points in a network the transceivers used for adding/dropping optical communication signals are connected to fixed wavelength, “coloured”, ports of the ROADM, and they are rigidly assigned to a particular ROADM input/output direction. Any wavelength and/or direction change for add and drop signals can be only performed by manually reconfiguring the WSS in the ROADM.
To implement colorless, directionless and contentionless ROADMs using the free space based 1×N WSS's as element switches, the only possible way is to interconnect with them a number of add/drop optical switches, splitters and tunable filters. In a fully flexible ROADM the number of expensive devices increases, the number of optical amplifiers also increases due to the high loss for signal distribution and switching, leading to an increase of ROADM cost, footprint and power consumption.
WO2012/123022 discloses a ROADM comprising three optical switch arrays: a cross-connect element comprising optical switches connected together as a first switch array; a drop element comprising optical switches connected together as a second switch array; and an add element comprising optical switches connected together as a third switch array. Each optical switch can switch a single wavelength on a grid of waveguides in order to perform all optical wavelength selective switching. An optical signal received at the cross-connect element may be routed to the drop element, where it is routed to a drop port, or may be routed to the add element, where it is routed to an output. An optical signal to be added is received at the add element where it is routed to an output.